This invention relates to a DC-to-AC voltage converter having galvanically separate input and output circuits, and comprising a converter transformer having at least one primary winding; a resonance capacitor connected to said at least one primary winding; a switching transistor of which the collector-emitter path is series connected to the at least one primary winding; and drive means for the switching transistor, wherein charge storage in the base-collector area of the switching transistor provides base drive for the switching transistor.
DC-to-AC voltage converters have many uses, such as the supply of power to gas discharge lamps or, after rectifying and smoothing the AC voltage, supplying power to electronic circuits, motors, relays, magnetic valves and clutches, etc.
An example of such a DC-to-AC voltage converter is disclosed in EP-B- No. 0105541. In said known DC-to-AC voltage converter the phenomenon of charge-storage in the base-collector area of the switching transistor is used to keep the switching transistor in its conductive state. Such a base drive by charge storage will automatically adapt to the load condition of the converter and increases the converter's reliability.
In order for such a converter to be used to supply a relatively high output power it is desirable that the maximum allowable collector current of the switching transistor can indeed be used. However, in that case additional controlled base drive for the switching transistor is necessary.
A From WO-A- No. 8704024 it is known in a DC-to-AC converter to provide proportional base drive for the switching transistor by means of a current transformer. Thereby the base drive current- collector current ratio will have a fixed value.
The use of the proportional base drive method, however, does not provide a solution to the problems resulting from the variation in the transistor parameter h.sub.fe and the variation in dynamic properties of switching transistors.
These problems do not occur when another known method is used for controlled base drive of switching transistors, i.e., comprising the use of a so-called Baker clamp circuit, sometimes referred to as a collector catcher. This known method is described in RCA Designer Handbook "Solid State Power Circuits" 1971, pp. 180-181. In accordance with this method, the base drive is reduced as soon as the collector-emitter voltage of the switching transistor is reduced below a given value. However, the use of the Baker clamp is accompanied with other drawbacks. First, when the Baker clamp is used, in switching off the switching transistor, a reverse recovery current will develop through the Baker clamp diode, which tends to bring the switching transistor back into the conducting state. Second, when the Baker-clamp is used, the base control may become unstable, especially with transistors having a high Vcbo value (e.g. of 1000 V or more). This is a result of the fact that it takes 1 to 5 microseconds for a reduction of the base current to result in a lower or higher collector-emitter voltage, while the charge stored in the base-collector junction has already changed. The result of the above facts is that the storage time is not constant but may vary greatly between different specimens of switching transistors. The storage time may also vary greatly depending upon the temperature of the switching transistors. Furthermore, when the base control is unstable, the storage time can vary greatly as a function of the period of conduction. This results in the collector current limitation being poorly defined. In fact, after a certain collector current has been detected, first the storage time lapses and only then the switching element passes into the non-conducting state, and meanwhile the collector current has increased further. As a consequence, the maximum power that can be given off, and also the peak voltage across the switching transistors in a converter of the type described in EP-B- No. 0105541 is subject to variation. In extreme cases, the converters may even become defect. Although the effect of the variations in characteristics of the switching transistors can in principle be reduced by suitable adjustment of the current detection level of the collector current limiting circuit arrangement, the temperature-dependence continues to exist. In addition, such an adjustment is difficult to perform in practice and constitutes a rather considerable cost item. Also, when the base control is unstable, as sometimes happens in Baker-clamp circuits, the control behaviour of the converter is sometimes seriously disrupted, as the storage time then varies periodically within a conducting period of the switching transistor. This may necessitate a lower maximum loop gain for the rest of the control loop. The control operation may even become fully unstable, because the total loop gain becomes very high with some settings. These control problems are comparable to those described in the publication entitled "Effects of Power Train Capacitance on the Dynamic Performance of DC-to-DC Converters Operating in the Discontinuous MMF Mode", in IEEE Transactions on Power Electronics, Vol. PE-2 No. 1, January 1987, pp. 2-19.
Furthermore, for bringing a switching transistor out of its conducting state, often a negative voltage of about 5V is applied through a current limiting element to the base of the switching transistor, because the saturation condition of the switching transistor is such that a high negative current must be drawn from the base. In the interior of the switching transistor, the base voltage in this period remains about 0.7 to 0.5V, but across the internal base series resistance of the switching transistor, such a high voltage drop develops that the application of an external negative voltage is needed. The generation of this voltage, however, complicates the circuit design and involves additional cost and also additional losses, which detract from the converter's efficiency.